Best Time to Buy and Sell Stock II

既然可以无数次地购买出售，那么之后后一天比前一天贵那就卖，否则留着。所以只需要从前往后扫一次，O（n）的复杂度。

int maxProfit(vector<int> &prices) {
    int n = prices.size();
    if(n == 0 || n == 1)
        return 0;
    int i;
    int previous=prices[0], latter;
    int result = 0;
    for(i = 1; i < n; i++){
        latter = prices[i];
        if(latter >= previous){
            result = result + latter - previous;
        }
        previous = latter;
    }
    return result;
}